Methods of Analysis by the U.S. Geological Survey 

National Water Quality Laboratory—Determination of 

Organophosphate Pesticides in Whole Water by Continuous 

Liquid-Liquid Extraction and Capillary-Column Gas 

Chromatography with Flame Photometric Detection

______________________________________________________________________

Water-Resources Investigations Report 03–4139

U.S. Department of the Interior

U.S. Geological Survey
Methods of Analysis by the U.S. Geological Survey 

National Water Quality Laboratory—Determination of 

Organophosphate Pesticides in Whole Water by Continuous 

Liquid-Liquid Extraction and Capillary-Column Gas 

Chromatography with Flame Photometric Detection

By Virendra K. Jha and Duane S. Wydoski

______________________________________________________________________

U.S. Geological Survey

Water-Resources Investigations Report 03–4139

U.S. Geological Survey Method O-3402-03

Denver, Colorado

2003
U.S. DEPARTMENT OF THE INTERIOR

Gale A. Norton, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of trade, product, or firm names in this report is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

_____________________________________________________________________________

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey U.S. Geological Survey

Chief, National Water Quality Laboratory Branch of Information Services

Box 25046, Mail Stop 407 Box 25286

Federal Center Federal Center

Denver, CO 80225-0046 Denver, CO 80225-0286
Contents iii

CONTENTS

Abstract .............................................................................................................................................................. 1

Introduction ......................................................................................................................................................... 1

Analytical method ............................................................................................................................................... 2

1. Scope and application ............................................................................................................................ 2

2. Summary of method............................................................................................................................... 2

3. Safety precautions .................................................................................................................................. 4

4. Interferences........................................................................................................................................... 4

5. Sampling methods, sample handling, preservation, and holding time................................................... 4

6. Apparatus and instrumentation .............................................................................................................. 4

7. Consumable materials ............................................................................................................................ 5

8. Calibration and quality-control standards and criteria........................................................................... 5

9. Analytical procedure .............................................................................................................................. 7

10. Gas chromatography with flame photometric detection analysis .......................................................... 8

11. Calculation of results ............................................................................................................................. 11

12. Reporting of results................................................................................................................................ 12

13. Method performance .............................................................................................................................. 14

Summary and conclusions................................................................................................................................... 25

References cited .................................................................................................................................................. 26

FIGURES

1. Photograph showing continuous liquid-liquid extractors with solvent-dispersing frits ......................... 4

2. Gas chromatogram with flame photometric detection of a 50-picrograms-per-microliter calibration 

standard solution of the individual organophosphate pesticides on a Restek Rtx-5 column for the 

analytical method .................................................................................................................................... 9

3. Gas chromatogram with flame photometric detection of a 50-picograms-per-microliter calibration 

standard solution of the individual organophosphate pesticides on a Restek Rtx-1701 column for the 

analytical method .................................................................................................................................... 10

TABLES

1. Method compound names, parameter codes, laboratory codes, and Chemical Abstracts Service 

registry numbers used by the U.S. Geological Survey National Water Quality Laboratory................... 3

2. Retention times of method compounds on the Rtx-5 and Rtx-1701 columns for the analytical method. 11

3. Suggested gas chromatography/flame photometry detection autosequence for the analytical method .. 11

4. Interim reporting levels for compounds in the analytical method........................................................... 12

5. Precision and bias of method compounds spiked at 0.02 microgram per liter in reagent-water, 

surface-water, and ground-water matrices combined and estimated method detection limit for 

the analytical method............................................................................................................................... 13

6. Precision and bias from eight determinations of the method compounds spiked at 0.02 microgram 

per liter in reagent water for the analytical method................................................................................. 15

7. Precision and bias from eight determinations of the method compounds spiked at 0.05 microgram 

per liter in reagent water for the analytical method................................................................................. 16
iv Contents

8. Precision and bias from six determinations of the method compounds spiked at 0.5 microgram 

per liter in reagent water for the analytical method.................................................................................. 17

9. Precision and bias from eight determinations of the method compounds spiked at 0.02 microgram 

per liter in surface water (South Platte River near Dartmouth St. and Platte River Dr., Denver, Colo.) 

for the analytical method.......................................................................................................................... 18

10. Precision and bias from eight determinations of the method compounds spiked at 0.05 microgram 

per liter in surface water (South Platte River near Dartmouth St. and Platte River Dr., Denver, Colo.) 

for the analytical method.......................................................................................................................... 19

11. Precision and bias from eight determinations of the method compounds spiked at 0.5 microgram 

per liter in surface water (South Platte River near Dartmouth St. and Platte River Dr., Denver, Colo.) 

for the analytical method.......................................................................................................................... 20

12. Precision and bias from eight determinations of the method compounds spiked at 0.02 microgram 

per liter in ground water (domestic well water, Evergreen, Colo.) for the analytical method ................. 21

13. Precision and bias from eight determinations of the method compounds spiked at 0.05 microgram 

per liter in ground water (domestic well water, Evergreen, Colo.) for the analytical method ................. 22

14. Precision and bias from eight determinations of the method compounds spiked at 0.5 microgram 

per liter in ground water (domestic well water, Evergreen, Colo.) for the analytical method ................. 23

15. Overall precision and bias combined from the determinations of the method compounds spiked at 

0.02, 0.05, and 0.5 microgram per liter in reagent water, surface water (South Platte River near 

Dartmouth St. and Platte River Dr., Denver, Colo.), and ground water (domestic well water, 

Evergreen, Colo.) for the analytical method ............................................................................................ 24

16. Compounds tested and found to have unacceptable performance ........................................................... 25
Contents v

CONVERSION FACTORS AND ABBREVIATED WATER-QUALITY UNITS

Degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) by using the following equation:

°F = (1.8 × °C) + 32.

Multiply By To obtain

Length

centimeter (cm) 3.94 x 10-1 inch

micrometer (mm) 3.94 x 10-5 inch 

millimeter (mm) 3.94 x 10-2 inch

meter (m) 3.281 foot 

nanometer (nm) 3.94 x 10-8 inch

Volume

liter (L) 2.64 x 10-1 gallon 

microliter (μL) 2.64 x 10-7 gallon 

milliliter (mL) 2.64 x 10-4 gallon

Mass

gram (g) 3.53 x 10-2 ounce, avoirdupois

microgram (μg) 3.53 x 10-8 ounce, avoirdupois

milligram (mg) 3.53 x 10-5 ounce, avoirdupois

nanogram (ng) 3.53 x 10-11 ounce

picogram (pg) 3.53 x 10-14 ounce, avoirdupois

Pressure

kilopascal (kPa) 1.45 x 10-1 pounds per square inch 

Concentration, in water

milligrams per liter (mg/L) 1 part per million (ppm)

micrograms per liter (μg/L) 1 part per billion (ppb)

nanograms per liter (ng/L) 1 part per trillion (ppt)
vi Contents

ABBREVIATIONS AND ACRONYMS

CCV continuing calibration verification standard

CLLE continuous liquid-liquid extraction

DZU diazolidinyl urea

EDTA ethylenediamine tetra acetic acid

FPD flame photometric detector

GC gas chromatograph

GC/FPD gas chromatograph/flame photometric detector

HPLC high-performance liquid chromatography

LRB laboratory reagent blank

LRS laboratory reagent spike

LS laboratory schedule

LT–MDL long-term method detection level

MDL method detection limit

mL/min milliliter per minute

MRL minimum reporting level

MSDS Material Safety Data Sheet

NWIS National Water Information System

NWQL National Water Quality Laboratory

OP organophosphate

pg/L picogram per liter

pg/μL picogram per microliter

QC quality control

RSD relative standard deviation

RT retention time

SOP standard operating procedure

SPE solid-phase extraction

TPC third-party check standard

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

μg/μL microgram per microliter

μg/L microgram per liter

< less than
Contents vii

GLOSSARY

Compound—A pesticide or pesticide degradate determined in an analysis.

Continuing calibration verification (CCV)—A calibration standard containing method compounds that is used to 

measure and control the bias of the existing calibration curve for the method compounds. The CCV is an 

instrumental standard only and is not processed through preparative steps of the method.

Fortified reagent-water-set sample—A quality-control sample prepared by adding known amounts of compounds 

to a reagent-water sample and analyzed with each set of environmental samples (usually 10). Also known as a 

“set spike.”

Laboratory reporting level (LRL)—The calculated concentration where the false-positive error is minimized to no 

more than 1 percent and the false-negative error is minimized to no more than 1 percent. The LRL is calculated as 

2 times the method detection limit. A compound determined to be not identified, confirmed, or measured in a 

sample is reported as <LRL.

Long-term method detection level (LT–MDL)—A detection level derived by determining the standard deviation 

of a minimum of 24 method detection limit spike-sample measurements over an extended time. LT–MDL data are 

collected continuously to assess year-to-year variations in the LT–MDL. The LT–MDL controls false positive 

error. The chance of falsely reporting a concentration at or greater than the LT–MDL for a sample that did not 

contain the compound is determined to be less than or equal to 1 percent.

Method detection limit (MDL)—The minimum concentration of a compound that can be measured and reported 

with 99-percent confidence that the compound concentration is greater than zero. At this concentration the false 

positive error is minimized to no more than 1-percent probability (U.S. Environmental Protection Agency, 1997).

Minimum reporting level (MRL)—Smallest measured concentration of a compound that may be reported reliably 

by using a given analytical method (Childress and others, 1999, p. 2).

Surrogate—A compound that is not expected to be found in any environmental sample and is added to every 

sample in a known amount prior to sample processing. The surrogate is used to monitor method performance for 

each sample.
Introduction 1

Methods of Analysis by the U.S. Geological Survey National 

Water Quality Laboratory—Determination of Organophosphate 

Pesticides in Whole Water by Continuous Liquid-Liquid 

Extraction and Capillary-Column Gas Chromatography with 

Flame Photometric Detection

By Virendra K. Jha and Duane S. Wydoski

Abstract

A method for the isolation of 20 parent 

organophosphate pesticides and 5 organophosphate 

pesticide degradates from natural-water samples is 

described. Compounds are extracted from water 

samples with methylene chloride using a continuous 

liquid-liquid extractor for 6 hours. The solvent is 

evaporated using heat and a flow of nitrogen to a 

volume of 1 milliliter and solvent exchanged to ethyl 

acetate. Extracted compounds are determined by 

capillary-column gas chromatography with flame 

photometric detection. 

Single-operator derived method detection limits in 

three water-matrix samples ranged from 0.003 to 0.009 

microgram per liter. Method performance was 

validated by spiking all compounds in three different 

matrices at three different concentrations. Eight 

replicates were analyzed at each concentration in each 

matrix. Mean recoveries of most method compounds 

spiked in surface-water samples ranged from 54 to 137 

percent and those in ground-water samples ranged 

from 40 to 109 percent for all pesticides. Recoveries in 

reagent-water samples ranged from 42 to 104 percent 

for all pesticides. The only exception was O-ethyl-Omethyl-S-propylphosphorothioate, which had variable 

recovery in all three matrices ranging from 27 to 79 

percent. As a result, the detected concentration of Oethyl-O-methyl-S-propylphosphorothioate in samples 

is reported in this method with an estimated remark 

code. Based on the performance issue, two more 

compounds, disulfoton and ethion monoxon, also will 

be reported in this method with an estimated remark 

code. Estimated-value compounds, which are “Ecoded” in the data base, do not meet the performance 

criteria for unqualified quantification, but are retained 

in the method because the compounds are important 

owing to high use or potential environmental effects 

and because analytical performance has been 

consistent and reproducible. 

INTRODUCTION

The U.S. Geological Survey (USGS) mission 

includes providing reliable scientific information for 

assessing the Nation's water resources. These 

assessments of the Nation's water include not only 

location, quantity, and availability, but also 

determinations of water quality, which require 

extensive and diverse studies along with supporting 

research. This part of the USGS mission produces 

much of the water-quality data used by planners, 

developers, water-quality managers, and agencies 

dealing with water-quality issues that require reliable, 

standardized data. 

Previously, the USGS National Water Quality 

Laboratory (NWQL) determined whole-water 

recoverable (method O-3104-83; NWQL laboratory 

schedules 1319, 1334, or 1399) and dissolved (method 

O-1104-83; NWQL laboratory schedule 1316, 

discontinued in 1997) organophosphate pesticides 

(OPs) by using the USGS methods described by 

Wershaw and others (1987, p. 27–31). These methods 

consisted of extracting either unfiltered or filtered 

water samples with hexane and analyzing the extracts 
2 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

by using packed-column gas chromatography with 

flame-photometric detectors (GC/FPD). In 1990, the 

packed-column technology was replaced by megabore 

fused-silica column technology (0.25-mm diameter). 

These original methods included only seven 

compounds (diazinon, ethion, malathion, methyl 

parathion, methyl trithion, parathion, and trithion). In 

1987, the NWQL offered the determination of five 

other organophosphate compounds: chlorpyrifos, 

tribufos, disulfoton, fonofos, and phorate as a custom 

add-on to the methods, and methyl trithion was 

dropped from the methods because a standard was no 

longer available. These five compounds became 

permanent (although undocumented) additions to the 

methods.

 The hexane extraction procedure used in these 

methods produced mean recoveries that were lower 

than desired. In addition, the recoveries for malathion 

and disulfoton had higher variability than desired. 

Various procedures were used to improve the 

recoveries for these two compounds, such as (1) adding 

iso octane to the sample extract as a keeper solvent, (2) 

adding ethyl acetate to the sample extract as a keeper 

solvent, (3) adding buffer solution to the sample, and 

(4) adding ascorbic acid to the sample. However, none 

has proven satisfactory to date (2003). To improve 

recoveries of organophosphate compounds, the NWQL 

developed a new method that uses a continuous liquidliquid extraction (CLLE) procedure that would 

improve organophosphate pesticide recoveries. This 

new method also expands the list of selected 

compounds from 11 to 25. Isofenfos is used as a 

surrogate standard because it is not expected to be 

found in water samples collected in the United States. 

 This method report addresses the following 

topics: principles and applications of the method, 

apparatus and consumable materials required, details 

of the calibration and analytical procedures, calculation 

of results, reporting of results (units and significant 

figures), and method performance. The method 

supplements other USGS methods for the 

determination of organophosphate pesticides that have 

been described by Wershaw and others (1987), 

Fishman (1993), Zaugg and others (1995), Sandstrom 

and others (2001), and Jha and Wydoski (2002, 2003). 

The new method was implemented as a custom method 

at the NWQL in October 2003. 

 There are substantial advantages of using the new 

method described in this report instead of the previous 

methods. The CLLEs provide high efficiency because 

they can be operated in batches. The new method is 

cost effective because it can be operated automatically 

after initial startup. 

ANALYTICAL METHOD

Organic Compounds and Parameter 

Codes: Organophosphate Pesticides, 

Whole Water, Gas Chromatography, 

O–3402–03 (see table 1)

1. Scope and Application

This method is suitable for the determination of 20 

parent organophosphate pesticides (OPs) and 5 related 

OP degradation products in whole-water samples (table 

1). Three of the compounds—disulfoton, ethion 

monoxon, and O-ethyl-O-methyl-S-propylphosphorothioate—are reported permanently with an 

estimated concentration because of variable 

instrumental and extraction stability problems. This 

method is applicable to the determination of pesticides 

and pesticide degradates that are (1) efficiently isolated 

from the sample matrix with methylene chloride 

extraction using a continuous liquid-liquid extractor, 

(2) chromatographically resolved and identified using a 

gas chromatograph (GC) equipped with flame 

photometric detectors (FPD), and (3) sufficiently stable 

to chemical or thermal degradation to allow accurate 

quantification by using all sampling and analysis steps 

of the method. 

Method compounds and their parameter codes, 

laboratory codes, and Chemical Abstracts Service 

(CAS) registry numbers for each compound are listed 

in table 1. 

2. Summary of Method

Reagent grade NaCl (60 g) is added to all field 

samples for preservation, including laboratory blank 

and spike samples. OPs and degradates are extracted 

from whole-water samples using CLLE, and methylene 

chloride is used as an extraction solvent. The extract is 

concentrated down to 4 to 6 mL on a heating mantle. 

The concentrate is further evaporated by using nitrogen 

to a volume of 1 mL. Finally, the extracts are solvent 

exchanged to ethyl acetate and analyzed by capillarycolumn GC/FPD with the external standard 

quantitation method.
Analytical Method 3

Table 1. Method compound names, parameter codes, laboratory codes, and Chemical 

Abstracts Service registry numbers used by the U.S. Geological Survey National Water 

Quality Laboratory 

[Compound marked with an asterisk (*) is reported permanently with an “E” code (estimated 

concentration) in method O-3402-03. CAS, Chemical Abstracts Service] 

Organophosphate compound 

(degradate indented) 

Parameter 

code 

Method 

code 

Laboratory 

code 

CAS 

number 

Chlorpyrifos 38932 B 4520 2921-88-2 

Diazinon 39570 E 4521 333-41-5 

Dimethoate 39009 A 4522 60-51-5 

Disulfoton * 39011 B 4523 298-04-4 

Disulfoton sulfone 62034 A 4524 2497-06-5 

Ethion 39398 C 4525 563-12-2 

Ethion monoxon * 62036 A 4526 17356-42-2 

Ethoprop 81758 A 4527 13194-48-4 

O-Ethyl-O-methyl-Spropylphosphorothioate * 

[O-Ethyl-O-meth_ioate]1

62026 A 4528 76960-87-7 

Fenthion 30006 A 4529 55-38-9 

Fonofos 82614 D 4530 944-22-9 

Fonofos oxygen analog 

(fonofos oxon)2

62031 A 4531 944-21-8 

Malathion 39530 C 4532 121-75-5 

Methidathion 62033 A 4533 950-37-8 

Methyl parathion 39600 C 4534 298-00-0 

Parathion 39540 C 4535 56-38-2 

Phorate 39023 B 4536 298-02-2 

Phorate oxygen analog 

(phorate oxon)2

62028 A 4537 2600-69-3 

Profenofos 62035 A 4538 41198-08-7 

Propetamphos 62032 A 4539 31218-83-4 

Sulfotepp 82201 A 4540 3689-24-5 

Sulprofos 62037 A 4541 35400-43-2 

Terbufos 82088 A 4542 13071-79-9 

Tribufos (DEF or S,S,STributylphosphotrithioate)2

39040 B 4543 78-48-8 

Trithion (Carbophenothion)2

 39786 C 4544 786-19-6 

 

Isofenfos (surrogate) 99577 A 4545 25311-71-1 

1

Name in bracket [ ] is an abbreviation used in the National Water Information System (NWIS) 

because of character number limitation. The short, 20-character name also was used in the tables of 

this report to minimize space taken by lengthy chemical names. The short name was defined as the 

first 14 characters and the last 5 characters of the name, joined by an underscore: “_”. 

2

Name in parentheses is alternative compound name. 
4 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

3. Safety Precautions

Proper laboratory safety procedures need to be 

followed when handling chemicals and operating 

equipment. Organophosphate compounds and 

especially the degradates in this method are recognized 

potent cholinesterase inhibitors. Liver function can be 

affected adversely or other health problems can occur 

from prolonged exposure. All appropriate safety 

equipment should be worn and extreme care exercised 

when handling these compounds and solvents. 

Appropriate clothing, nitrile gloves, and eye protection 

are worn, and adequate ventilation is used when 

preparing samples or standard solutions. It is important 

to read the Material Safety Data Sheet (MSDS) on each 

compound and solvent prior to using this method. All 

organic solvents, water samples, and rinse waste are 

disposed in accordance with local hazardous wastedisposal rules and regulations.

4. Interferences

There are many organophosphate compounds in 

natural matrices that GC/FPD will detect. This method 

is designed to minimize false positives through dual 

GC column confirmation. Mass-spectral confirmation 

also should be used to confirm identification, if 

uncertain. Sulfur and organosulfur compounds and 

unknown organophosphate compounds occasionally 

might interfere with qualification and quantification of 

other individual organophosphate compounds.

5. Sampling Methods, Sample Handling, 

Preservation, and Holding Time

Detailed descriptions of sampling methods used by 

the USGS for obtaining depth- and width-integrated 

surface-water samples, sampling methods for 

obtaining ground-water samples, and sample 

processing are described by Wilde and others (1999). 

Samples are collected in pre-cleaned and baked 1-L, 

33-mm neck, amber glass bottles with Teflon-lined 

screw caps and shipped chilled by overnight mail to the 

laboratory. After the samples are logged into the 

laboratory information management system (LIMS), 

60 g of NaCl is added to the samples for preservation 

and they are stored at 4oC until ready for extraction 

(usually within 7 days). No sample or extract holdingtime studies have been performed for this method. 

However, holding-time studies in reagent water and on 

dry solid-phase extraction (SPE) have been performed 

by Sandstrom and others (2001) for laboratory 

schedule (LS) 2002 method, which is used to determine 

most of the compounds in this new method. Based on 

findings by Sandstrom and others (2001), most of the 

compounds in this method have a holding time of 8 

days or longer. Winslow and others (2001) have shown 

that the addition of chemical preservatives is required 

to preserve selected OPs. They added DZU as an 

antimicrobial inhibitor, Na3 EDTA to prevent metal 

ion-induced hydrolysis, and buffered the samples to 

pH 7. Winslow and others (2001) obtained acceptable 

recoveries in U.S. Environmental Protection Agency 

(USEPA) method 526, a method that uses gas 

chromatography/mass spectrometry and SPE. Some 

OPs might be susceptible to hydrolysis because no 

chemical preservatives are added to the sample. 

6. Apparatus and Instrumentation

6.1 Analytical balance, capable of accurately 

weighing to the nearest 0.00001g.

6.2 Continuous liquid-liquid extractors (CLLE), 

including extended extraction chamber, a coarse frit 

reflux dispersion apparatus, a condenser (made by 

Allen Scientific Glassware, Inc. or equivalent), 

stopcock, and a 25-mL receiver with holding clip 

(see fig. 1).

Figure 1. Continuous liquid-liquid extractors with solventdispersing frits.
Analytical Method 5

6.3 Variac variable transformer, 0 to 140 volts, or 

equivalent.

6.4 Tube or teardrop-shaded heating mantle.

6.5 A 25-mL calibrated receiver with 1-mL 

collection nipple and ground glass fitting, cleaned and 

baked.

6.6 A 100-μL microdispenser or other volume as 

needed.

6.7 Waste containers, 1-L, resistant to 

chlorinated solvents.

6.8 Waste containers, 4-L, resistant to 

chlorinated solvents.

6.9 Nitrogen gas solvent evaporation device, 

Organomation Associates, Inc. or equivalent.

6.10 GC/FPD, Hewlett-Packard (HP), Model 

5890 GC or comparable with HP 7673A automated 

sample injector. Dual HP flame photometric detectors 

or equivalent, and a data system with Turbochrom 

chromatography data-acquisition software and Target 

data-processing software or equivalent.

7. Consumable Materials

7.1 Glass bottles, amber, 1-L, 33-mm neck, 

baked at 450oC for 2 hours, fitted with Teflon-lined 

screw caps; NWQL glass-chilled container (GCC) or 

equivalent.

7.2 Reagent water, ultrapure, B&J brand for 

HPLC or equivalent.

7.3 Ethyl acetate, methylene chloride and 

acetone solvents, glass distilled, pesticide analysis 

grade, B&J or equivalent.

7.4 Boiling chips, four-mesh granule sizes, 

hexane rinsed, baked at 400oC for 4 hours.

7.5 Sodium chloride, reagent grade, baked at 

400oC for 4 hours.

7.6 OP surrogate solution.

7.7 OP spike solution.

7.8 Microdispenser glass bores, 100-μL, baked at 

400oC for 4 hours.

7.9 Pasteur pipets (disposable), with rubber 

bulbs.

7.10 Vial, 1.5- or 2-mL, amber glass, with 

aluminum crimp caps that have dual polytetrafluoroethylene (PTFE)-faced silicon rubber septa or 

open-top screw-cap with PTFE-faced silicon rubber 

septum, Supelco Inc. or equivalent.

8. Calibration and Quality-Control 

Standards and Criteria 

All quality-control (QC) information needs to be 

evaluated to determine whether analytical data are of 

acceptable quality to be reported. Minimum QC 

requirements include the following: (1) analysis of 

laboratory reagent blank (LRB); (2) determination of 

surrogate standard compound recoveries in each 

sample, LRB, and laboratory reagent spike (LRS); (3) 

determination of compound recoveries in the LRS; and 

(4) assessment of the GC/FPD chromatographic 

performance. 

8.1 Calibration standards. Stock standards for 

the pesticides and degradates were obtained as pure 

materials from the USEPA National Pesticide Standard 

Repository (Ft. Meade, Md.) or commercial vendors 

(ChemService; EQ Laboratories). The analytical range 

for this method is from 0.005 to 0.100 μg/L. Calibration 

standards are prepared at six different concentrations 

(5, 10, 20, 50, 80, and 100 pg/μL) for each compound 

and surrogate compound by adding known volumes of 

stock standard solutions to a volumetric flask. The 

resulting solution is diluted to volume with ethyl 

acetate. The lowest standard needs to represent 

compound concentrations near, but greater than, its 

respective method detection limit (MDL). The 

remaining standards need to bracket the compound 

concentrations expected in the sample extracts. 

8.2 Calibration curve. Starting with the lowest 

concentration, each calibration standard is analyzed 

and response is tabulated (peak area) in relation to the 

concentration in the standard. The results are used to 

prepare a linear calibration curve for each compound. 

For each sample set all six calibration standard 

solutions are analyzed prior to analyzing the samples. 

The determined concentration should be ±20 percent of 

the expected concentration for all compounds. The 

correlation coefficient (r2) for the calibration curve 

regression needs to be equal to or greater than 0.995. If 

the instrument does not meet these calibration criteria, 

the problem is corrected by servicing the GC or by 

preparing and reanalyzing new calibration standards.

8.3 Surrogate standard solution. The surrogate 

standard solution is prepared with isofenfos, which is 

available through Absolute Standard Inc. or equivalent. 

Surrogate solution is prepared by adding 250 μL of 

isofenfos stock solution (100 μg/mL in hexane) into 

25 mL of methanol. The final concentration of 

isofenfos in methanol is 1,000 pg/μL. The surrogate 

standard solution is added to the sample at the time of 
6 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

extraction, 100 μL of the surrogate standard is added to 

1 L of each field sample and to the LRS and LRB, and 

used to monitor performance of the sample preparation 

procedure (M.R. Burkhardt and T.J. Maloney, U.S. 

Geological Survey, written commun., 1998). Standard 

statistical techniques are used to establish control limits 

for surrogate recovery. When surrogate recovery for a 

sample is greater than the upper or less than the lower 

control limits, the following are checked: (1) 

calculations, so as to locate possible mathematical 

errors; (2) spiking or calibration solutions for possible 

surrogate (and other compounds) degradation; (3) 

contamination, which usually produces positive bias; 

and (4) instrument performance (see section 8.8). If 

those steps do not reveal the cause of the problem, the 

extract is reanalyzed. If a set blank extract reanalysis 

fails the surrogate control-limit criteria (M.R. 

Burkhardt and T.J. Maloney, U.S. Geological Survey, 

written commun., 1998), then the problem needs to be 

identified and corrected before continuing the analysis. 

If sample extract reanalysis meets the surrogate 

recovery and other QC criteria, then the result is 

reported using the reanalyzed extract data. If the 

surrogate in sample extract fails the recovery criteria, 

then protocol for corrective action is followed, which 

includes assigning estimated (E-code) qualifier, raising 

the sample reporting level, or not reporting the sample 

data, depending on the nature of the surrogate failure.

8.4 Spike solution. The LRS solution is prepared 

in methanol by adding 125 μL of an organophosphate 

stock (100 μg/mL in hexane) into 25 mL methanol. 

This solution contains all of the organophosphate 

compounds of interest, except the current (2003) 

surrogate compound (isofenfos). The spike solution 

concentration is 500 pg/μL, and 100 μL of this solution 

is added to 1 L of reagent water to prepare the LRS. The 

LRS is used to monitor recovery efficiencies for all 

method compounds (M.R. Burkhardt and T.J. Maloney, 

U.S. Geological Survey, written commun., 1998). For 

this report, spike solution was added to spike samples 

at three different concentrations (0.02, 0.05, and 

0.5 μg/L) for the method performance determinations. 

The laboratory needs to analyze at least one LRS 

sample with every 10 samples or one per sample set (all 

samples extracted within a 24-hour period), whichever 

is greater. The concentration of each compound in the 

LRS sample needs to be within the range of the 

calibration standards. Standard statistical techniques 

(M.R. Burkhardt and T.J. Maloney, U.S. Geological 

Survey, written commun., 1998) are used to establish 

control limits for compound recovery for the LRS. If 

the recovery of any compound falls outside the controllimit criteria, that compound is judged out of control, 

and the source of the problem needs to be identified and 

resolved before continuing the analyses. The data for 

compounds that fail quality-control criteria need to 

follow corrective-action procedures, which include 

assigning an “estimated” remark code, or raising the 

sample reporting level, or not reporting the sample 

data, depending on the nature of the spike failure.

The laboratory periodically needs to determine and 

document its detection capabilities for the method 

compounds. The detection levels for this method will 

be evaluated continuously using the long-term method 

detection level (LT–MDL) procedure (Childress and 

others, 1999) or other procedure as adopted by the 

NWQL. 

8.5 Third-party check (TPC) standard. The 

third-party check standard is a separate source material 

from the standards. The TPC contains all of the OP 

compounds of interest, except the current (2003) 

surrogate compound (isofenfos). A working TPC 

standard is prepared in ethyl acetate by adding 10 μL of 

the TPC stock standard (100 μg/mL in hexane) to 10 

mL ethyl acetate. The final working concentration of 

the TPC is 100 pg/μL. The TPC is analyzed in each 

analytical sequence after the calibration standards to 

verify the calibration curve and is compared to the 

calibrated standard. The determined concentration for 

all compounds in the TPC standard needs to be ±30 

percent of the expected concentration.

8.6 Continuing calibration verification (CCV). 

The continuing calibration verification standard 

concentration typically is at the midpoint of the 

calibration range, usually the 20- or 50-pg/μL OP 

standard. A 20- or 50-pg/μL calibration standard 

containing all of the method compounds is inserted in 

an autosampler vial and placed after every 10 field or 

QC samples throughout the GC analytical sequence. 

The CCV standard is used to monitor the calibration of 

the GC for precision and bias. The calculated 

concentration in the CCV needs to be within 20 percent 

of the expected concentration for each compound. If 

the determined concentrations of compounds in the 

CCV are outside these control limits, corrective action 

needs to be taken. Typical corrective action in this 

instance is to maintain the instrument and return it to 

acceptable performance. This might require 

recalibration. In addition, the environmental samples 
Analytical Method 7

need to be reanalyzed (M.R. Burkhardt and 

T.J. Maloney, U.S. Geological Survey, written 

commun., 1998).

8.7 Laboratory reagent blank (LRB). Before 

processing any samples, the analyst must demonstrate 

that all glassware and reagent interferences are under 

control. Each time a set of samples is extracted, a LRB 

needs to be analyzed with representative matrix and all 

reagents used in the procedure. If the LRB contains 

interfering peaks that would prevent the determination 

of one or more compounds at the MDL, then the source 

of contamination is determined and the interference is 

eliminated before continuing future sample processing 

and analysis (M.R. Burkhardt and T.J. Maloney, U.S. 

Geological Survey, written commun., 1998).

8.8 Instrument system. Instrument performance 

needs to be monitored daily at a minimum. Gas 

chromatographic performance normally is reflected in 

the variation of determined concentration of the 

selected compound in calibration standards, TPC, and 

CCVs relative to the concentrations obtained by using 

a new capillary column and freshly prepared standards. 

Failure to meet the calibration, TPC, or CCV criteria 

indicates that GC maintenance is required to bring the 

system into compliance. A portion of the guard column 

might be cut off and removed to restore performance, 

or the injection port liner might be replaced. 

8.9 Other GC/FPD performance requirements. 

Sample extract concentrations that exceed the high 

concentration calibration standard need to be diluted to 

within the calibration range and reanalyzed. 

The laboratory might adopt additional QC 

practices for use with this method (see Pirkey and 

Glodt, 1998). The specific practices that are most 

productive depend on the needs of the laboratory and 

the nature of the samples. 

9. Analytical Procedure

9.1 The CLLE apparatus, receivers, and stoppers 

for each sample are triple rinsed (first rinse with 

reagent water, second with acetone, and third with 

methylene chloride). Precleaned Teflon stopcocks are 

attached to the CLLE apparatus in the closed position. 

Two to five boiling chips are placed in the bottom of 

each receiver and attached firmly to the extractor with 

a clip.

9.2 The sample bottle containing the sample is 

weighed to the nearest gram and recorded (W1) (see 

section 11.2, equation 2).

9.3 Two additional bottles are prepared for each 

set of 10 samples, each containing about 1,000 mL of 

reagent water for the LRB and the LRS. Sixty g of 

NaCl is added to LRB and LRS, 100 μL (500 pg/μL) of 

primary fortification (spike) is added into reagent water 

spike bottle, and 100 μL (1,000 pg/μL) of surrogate 

solution is added to each sample, including LRB and 

LRS. Each sample bottle is capped and shaken well to 

mix until all the salt dissolves.

9.4 Fifty mL of methylene chloride is added to 

each CLLE extractor. Each sample is slowly 

transferred into an extractor using the side access arm. 

A clean stainless steel funnel is used to facilitate this 

transfer. About 10 mL of reagent water is transferred 

into the sample bottle. The bottle is capped and shaken 

to loosen any particulate matter adhered to the sample 

bottle. The water and particulate matter are transferred 

to the CLLE extractor. The sample bottle is rinsed with 

about 10 mL of methylene chloride, shaken or swirled 

to ensure the entire surface of the inside of the sample 

bottle has contact with solvent. Solvent is dispensed 

into CLLE. The solvent rinse procedure is performed 

twice. And finally, the sample bottle is rinsed with 

reagent water to ensure all remaining methylene 

chloride from earlier rinse is removed from sample 

bottle and transferred to CLLE.

9.5 With the frit assembly raised out of the way, 

reagent water is added to the extractor until methylene 

chloride just begins to drip from the CLLE side arm 

into the 25-mL receiver. The frit assembly is lowered 

until the bottom of the frit touches the surface of the 

water. 

9.6 Methylene chloride is added with Teflon 

squeeze bottle into the spout at the top of the condenser 

until the reservoir above the frit is filled. Methylene 

chloride will begin to drip from the frit, causing the 

receiver to fill with methylene chloride. This procedure 

is continued until there is about 15 to 20 mL of 

methylene chloride in the receiver.

9.7 The water to the condensers is turned on, and 

the side access arm closed with a glass stopper. The 

heating mantle is placed over the receiver and hooked 

into place. The Variac voltage controllers are turned on 

and set for 70 volts. The methylene chloride should boil 

vigorously in the receiver tube and extraction continue 

for 6 hours. After the extraction is complete, the CLLE 

is drained into a designated waste container and the 

methylene chloride continues to boil in the receiver 

tube until the level reaches 4 to 6 mL. The receiver tube 

is allowed to cool.
8 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

9.8 The empty sample bottle is weighed and 

recorded (W2). W2 is subtracted from W1 to obtain the 

exact volume of sample (W) extracted (weight = 

volume) (see section 11.2, equation 2).

9.9 The extract in the receiver tube is evaporated 

with a gentle stream of nitrogen until the volume of the 

sample is 1 mL. Solvent exchange to ethyl acetate is 

done by adding 1 mL of ethyl acetate into the receiver 

and the solution is evaporated with nitrogen down to 

1 mL. Repeat the solvent exchange and evaporation 

with nitrogen. A final volume of 1 mL needs to be 

achieved after the second nitrogen evaporation. The 

final extract is transferred to a 1.8-mL autosampler 

vial. The autosampler vial is capped and stored in a 

refrigerator at 4oC until ready for analysis.

10. Gas Chromatography with Flame 

Photometric Detection Analysis

10.1 The sample extracts are analyzed by gas 

chromatography with flame photometric detection 

(GC/FPD) by using a dual capillary-column system 

equipped with an autosampler; one split/splitless 

injection port (operated in the splitless mode); a 1-m, 

0.32-mm inside-diameter (ID) section of fused silica 

capillary tubing, uncoated, deactivated guard column; a 

Y-type column connector to connect the guard column 

to the primary and secondary capillary columns; and 

two flame photometric detectors. A computer system is 

used to control the autosampler, GC operational 

conditions, and to acquire and process responses from 

the dual detectors.

10.2 The gas chromatographic configuration is 

described in this section.

Column 1 (primary column): 30-m long by 

0.25-mm ID, 5 percent diphenyl and 95 percent 

dimethyl polysiloxane bonded fused silica capillary 

column, 0.25-μm film thickness (Restek Corp. Rtx-5 or 

equivalent).

Column 2 (confirmation column): 30-m long by 

0.25-mm ID, 14 percent cyanpropylphenyl and 86 

percent dimethyl polysiloxane bonded fused silica 

capillary column, 0.25-μm film thickness (Restek 

Corp. Rtx-1701 or equivalent).

Carrier gas: Helium, 99.999 percent purity, 1 to 3 

mL/min column flow. This flow range corresponds to a 

linear flow velocity of 20 to 40 cm/sec on a Van 

Deemter plot, when using 30-m by 0.25-mm ID 

columns.

Detector make-up gas: Nitrogen, 99.999 percent 

purity, 4 to 10 mL/min flow.

Detector gas: Hydrogen, 99.999 percent purity, 3 to 

5 mL/min flow. 

Air: 99.6 percent purity, 90 to 110 mL/min flow.

Injection mode: Splitless, injection port sweep 

30 mL/min. Column head pressure 138 kPa (20 lb/in2 ).

Septum purge rates: 1 to 2 mL/min. Purge valves 

are on (open) at 2 minutes and off (closed) for 2 

minutes prior to the end of sample analysis. Both 

columns are connected to guard column with a “Y” 

splitter, and the guard column is connected to the 

injection port. If flows through the GC columns are 

equivalent, then an injection volume of 4 μL of extract 

is divided evenly onto both columns.

Injector temperature: 220oC

Detector temperature: 220oC

Detectors: Two flame photometric detectors (FPD) 

are used, set for phosphorus “P” mode, with optical 

filters that transmit 525-nm wavelength for specific 

phosphorus response.

Oven temperature program—Initial temperature 

60oC, hold 1 minute.

Ramp 1—15oC/min to 160oC, hold 0 minute

Ramp 2—1oC/min to 186oC, hold 0 minute

Ramp 3—7oC/min to 280oC, hold for 7 minutes. 

Total analysis time is about 54 minutes.

10.3 Determine compound retention times 

(RT)—Following GC setup, compound retention times 

are established by using the calibration standard 

solutions. A typical separation and peak shape obtained 

using the GC operating conditions described in section 

10.2 for the individual OP pesticides on the Rtx-5 

column are shown in figure 2. Separation and peak 

shape on the Rtx-1701 column are shown in figure 3. 

Peak identifications and retention times are listed in 

table 2 for the method compounds on the Rtx-5 and 

Rtx-1701 columns shown in figures 2 and 3.

10.4 Coelution problems—Two coelutions (one 

with fonofos and propetamphos and a second with 

fenthion, chlorpyrifos, and parathion) were observed 

on the Rtx-5 column, and three coelutions (one with 

chlorpyrifos and methyl parathion, a second with 

methidathion and profenophos, and a third with 

disulfoton sulfone, ethion, and sulprofos) were 

observed on the Rtx-1701 column using the GC 

conditions described in section 10.2. Compounds with 

coelutions on one column are well separated from 

method compounds on the other column except for 

chlorpyrifos (see table 2). Coelution conditions require 
 Analytical Method 9

Figure 2. Gas chromatogram with flame photometric detection of a 50-picograms-per-microliter calibration standard solution of the individual organophosphate pesticides on a Restek Rtx-5 column for the analytical method. Compound identifications are listed in table 2. Chromatographic conditions are given in the text.

O-Ethyl-O-meth_ioate

Phorate oxon

Ethoprop

Sulfotepp

Phorate

Fonofos oxon

Dimethoate

Terbufos

Fonofos

Propetamphos

Diazinon

Disulfoton

Methyl parathion

Malathion

Fenthion

Chlorpyrifos

Parathion

Isofenfos (surrogate)

Methidathion

Disulfoton sulfone

Profenofos

Tribufos

Ethion monoxon

Ethion

Sulprofos

Trithion

8.99

14.37

14.81

16.37

16.68

16.88

18.03

19.65

19.85

19.96

20.77

21.04

25.05

29.47

30.23

30.40

30.52

35.37

36.68

37.61

38.93

39.15

39.97

41.72

42.23

42.60

RESPONSE, IN MILLIVOLTS

TIME, IN MINUTES
 10 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID 

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION Figure 3. Gas chromatogram with flame photometric detection of a 50-picograms-per-microliter calibration standard solution of the individual organophosphate pesticides on a Restek Rtx-1701 column for the analytical method. Compound identifications are listed in table 2. Chromatographic conditions are given in the text.

O-Ethyl-O-meth_ioate

Phorate oxon

Ethoprop

Sulfotepp

Phorate

Fonofos oxon

Dimethoate

Terbufos

Fonofos

Propetamphos

Diazinon

Disulfoton

Malathion

Fenthion

Methyl parathion

Parathion

Isofenfos

(surrogate)

Methidathion

Disulfoton sulfone

Tribufos

Ethion monoxon

Trithion

9.23

17.21

16.80

19.21

18.75

20.48

28.39

21.84

22.81

25.40

22.57

24.20

36.31

35.71

33.96

Chlorpyrifos 33.85

37.76

39.00

41.01

Profenofos 41.21

43.67

40.49

43.15

44.24

Ethion 43.79

Sulprofos 43.87

RESPONSE, IN MILLIVOLTS

TIME, IN MINUTES
Analytical Method 11

special identification and calibration considerations. 

Compounds that show coelution with another method 

compound or interference on one column must be 

quantified on the other column, where no coelution 

problems occur. 

10.5 GC autosequence—The suggested sequence 

for an automated analysis is listed in table 3.

10.6 Gas chromatography/flame photometric 

detection compound calibration—This method is an 

external standard quantitation method that uses 

multipoint external standard calibration for singlecomponent compounds. The GC/FPD is calibrated 

(and compounds subsequently quantitated; see section 

11) by using results obtained on both capillary 

columns. The individual pesticides can be calibrated by 

multipoint curves produced from analysis of the 5- to 

100-pg/μL calibration standards (8.1). The GC/FPD 

peak area for the compound (A1) is plotted in relation 

to the mass (in picograms) of the compound (C1) for 

each of the 5- to 100-pg/μL calibration standards 

injected. A calibration curve for this plot is calculated 

by using the simple linear regression equation (1) in 

section 11.1.

Table 3. Suggested gas chromatography/flame photometry 

detection autosequence for the analytical method

[pg/μL, picograms per microliter]

11. Calculation of Results

11.1 Calibration standards are injected and the 

peak area of compound (A1) and concentration of 

compound (C1) in each calibration standard, in 

picograms per microliter, are tabulated. A1 is plotted in 

relation to C1V1 from results of equation 1:

 where

V1 = volume of calibration standard injected, 

in microliters;

m = slope of regression curve, in area per

picograms; and

b = y-intercept of regression curve.

11.2 Samples are injected and the peak area 

response for identified compounds in the sample is 

determined. The concentration of the compound in the 

Table 2. Retention times of method compounds on 

the Rtx-5 and Rtx-1701 columns for the analytical method 

[Compounds listed in Rtx-5 retention time order] 

Retention time (minutes) Compound Rtx-5 Rtx-1701 

O-Ethyl-O-meth_ioate 8.99 9.23 

Phorate oxon 14.37 17.21 

Ethoprop 14.81 16.80 

Sulfotepp 16.37 19.21 

Phorate 16.68 18.75 

Fonofos oxon 16.88 20.48 

Dimethoate 18.03 28.39 

Terbufos 19.65 21.84 

Fonofos 1

19.85 22.81 

Propetamphos 1

19.96 25.40 

Diazinon 20.77 22.57 

Disulfoton 21.04 24.20 

Methyl parathion 25.05 5

33.96 

Malathion 29.47 36.31 

Fenthion 230.23 35.71 

Chlorpyrifos 2

30.40 533.85 

Parathion 2

30.52 37.76 

Isofenfos (surrogate) 35.37 39.00 

Methidathion 36.68 4

41.01 

Disulfoton sulfone 37.61 3

43.67 

Profenofos 38.93 4

41.21 

Tribufos 39.15 40.49 

Ethion monoxon 39.97 43.15 

Ethion 41.72 3

43.79 

Sulprofos 42.23 3

43.87 

Trithion 42.60 44.24 

1

Coelutions on Rtx-5, well separated on Rtx-1701. 

2

Coelutions on Rtx-5, well separated on Rtx-1701. 

3

Coelutions on Rtx-1701, well separated on Rtx-5. 

4

Coelutions on Rtx-1701, well separated on Rtx-5. 

5

Coelutions on Rtx-1701, well separated on Rtx-5. 

Standard or sample type 

Ethyl acetate gas chromatograph injection blank 

Calibration standard 5 pg/μL 

Calibration standard 10 pg/μL 

Calibration standard 20 pg/μL 

Calibration standard 50 pg/μL 

Calibration standard 80 pg/μL 

Calibration standard 100 pg/μL 

Third-party check solution 

Laboratory reagent blank (LRB) 

Laboratory reagent spike (LRS) 

Ten field samples 

Continuing calibration verification (CCV) standard (50 pg/μL) 

LRB 

LRS 

Ten field samples 

CCV standard (50 pg/μL) 

LRB 

LRS 

Ten field samples 

CCV standard (50 pg/μL)

A1 mC1V1 = + b , (1) 
12 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

sample is calculated by rearranging equation (1) and 

including other conditions, in equation 2:

where

C2 = concentration of the compound in the

sample, in picograms per microliter;

A2 = peak area of the compound in the sample;

V2 = final volume of the sample extract prior to

injection into GC, in milliliters; 

V3 = volume of extract injected, in microliters

[NOTE: V3 = V1 (equation 1) because 

4 μL is injected using an autosampler];

V4 = volume of sample extracted by CLLE, in

milliliters, equals weight (W) of sample

extracted by CLLE (assuming 

1 mL = 1 g);

where

W1 = weight of sample +bottle before extraction;

W2 = weight of sample bottle after extraction; and

DF = dilution factor.

11.3 Surrogate and spike recoveries are calculated 

in percent, as follows:

 

where

Cs = concentration of the surrogate standard

(section 8.3) or spike (section 8.4)

solution, in picograms per microliter; and

Vs = volume of surrogate (100 μL) or spike 

100 μL) solution added.

12. Reporting of Results

 It is the policy of NWQL to report dual column 

organic analysis in a conservative manner. Generally, 

the column that produces the lower concentration 

during calibration is used to report the analytical 

results. Therefore, the quantitative value that is 

reported is column dependent. If coelution problems 

exist, the column that has least interference would be 

selected for quantitation. Compound concentrations in 

field samples are reported in micrograms per liter 

(μg/L). If the concentration is less than the lowest 

calibration standard, the concentration is reported to 

two significant digits after the decimal place, and the 

“E” code is used to indicate that it has been estimated. 

If the concentration is greater than the highest 

calibration standard, the extract is diluted with ethyl 

acetate to bring the concentration within calibration 

range and the concentration is reported to two 

significant digits after the decimal place. Surrogate 

data for all sample types are reported in percent 

recovered. The LRS results are reported in percent 

recovered. Interim reporting levels at twice the MDL 

for all compounds in this method are listed in table 4, 

except for O-ethyl-O-methyl-S-propylphosphorothioate, which has been raised to four times the MDL 

because of variable recoveries. Estimates of MDLs 

using the procedures outlined by the U.S. Environmental Protection Agency (1997) are listed in table 5. 

C2

A2 ( ) – b

mV3

------------------- V2

V4

= × ----- × DF , (2)

V4 W W1 W2 = = – , 

C2

C

s

V

s

× × 100

V4

= ---------------------------------- , (3)

Table 4. Interim reporting levels for compounds 

in the analytical method 

[(μg/L), microgram per liter] 

Compound 

Interim 

reporting levels 

(μg/L) 

Chlorpyrifos 0.006 

Diazinon .016 

Dimethoate .012 

Disulfoton1

 .018 

Disulfoton sulfone .018 

Ethion .008 

Ethion monoxon1 .012 

Ethoprop .008 

 

O-Ethyl-O-meth_ioate1 .016 

Fenthion .010 

Fonofos .008 

Fonofos oxon .014 

 

Malathion .008 

Methidathion .012 

Methyl parathion .008 

Parathion .008 

 

Phorate .010 

Phorate oxon .012 

Profenofos .010 

Propetamphos .006 

 

Sulfotepp .006 

Sulprofos .008 

Terbufos .008 

Tribufos .008 

Trithion .008 

1

Compound will be reported permanently as “E” 

coded (estimated concentration) in this method. 
Analytical Method 13

Table 5. Precision and bias of method compounds spiked at 0.02 microgram per liter in reagentwater, surface-water, and ground-water matrices combined and estimated method detection limit 

for the analytical method (data for individual matrices are provided in subsequent tables)

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter] 

Compound 

Number 

of 

observations 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of true 

conc.) 

Method 

detection 

limit 

(μg/L) 

Chlorpyrifos 24 0.015 0.0014 8.95 76.67 0.0034 

Diazinon 24 .016 .0030 18.46 81.04 .0075 

Dimethoate 24 .021 .0025 12.10 105.21 .0064 

Disulfoton1 24 .014 .0038 27.40 68.75 .0094 

 Disulfoton sulfone 24 .023 .0035 15.17 116.46 .0088 

 

Ethion 24 .015 .0017 11.32 76.88 .0043 

 Ethion monoxon1 24 .021 .0026 12.48 102.29 .0064 

Ethoprop 24 .016 .0014 8.72 80.21 .0035 

 O-Ethyl-O-meth_ioate1 16 .015 .0017 11.59 73.44 .0044 

Fenthion 24 .015 .0019 12.83 74.17 .0048 

 

Fonofos 24 .015 .0014 9.36 76.67 .0036 

 Fonofos oxon 24 .020 .0028 14.00 99.17 .0069 

Malathion 24 .018 .0016 8.82 90.00 .0040 

Methidathion 24 .020 .0025 12.16 102.08 .0062 

Methyl parathion 24 .018 .0015 8.57 88.54 .0038 

 

Parathion 24 .015 .0016 10.59 75.83 .0040 

Phorate 24 .014 .0018 13.42 68.33 .0046 

 Phorate oxon 24 .015 .0022 14.51 75.83 .0055 

Profenofos 24 .019 .0020 10.19 96.46 .0049 

Propetamphos 24 .015 .0014 9.01 75.00 .0034 

 

Sulfotepp 24 .014 .0012 8.93 67.92 .0030 

Sulprofos 24 .015 .0018 12.20 72.50 .0044 

Terbufos 24 .014 .0017 12.28 71.04 .0044 

Tribufos 24 .016 .0016 10.26 80.00 .0041 

Trithion 24 .016 .0016 9.98 80.21 .0040 

1

Compound will be reported permanently as “E” coded (estimated concentration in this method. 
14 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

13. Method Performance

13.1 Samples of (1) reagent water, (2) surface 

water collected from the South Platte River, near 

Dartmouth Street and Platte River Drive, Denver, 

Colo., and (3) ground water collected from a domestic 

well in Evergreen, Colo., were used to test method 

performance. Eight samples of each water type were 

fortified with each compound at three different concentrations of 0.02, 0.05, and 0.5 μg/L. One sample for 

each water type was unfortified to determine any 

potential background contamination or interference in 

each matrix.

13.2 All samples for a given matrix were extracted 

on the same day. Extracts were analyzed by GC/FPD, 

but different concentrations and matrices were 

analyzed on different days. Mean recoveries of most 

method compounds spiked in surface-water samples 

ranged from 54 to 137 percent and those in groundwater samples ranged from 40 to 109 percent for all 

pesticides. Recoveries in reagent-water samples ranged 

from 42 to 104 percent for all pesticides. The only 

exception was O-ethyl-O-methyl-S-propylphosphorothioate, which had variable recovery in all three 

matrices ranging from 27 to 79 percent. Precision and 

bias data are listed in tables 6 through 14.

13.3 The unfortified surface-water samples 

contained low concentrations of diazinon (0.008 μg/L). 

This concentration (0.008 μg/L) was subtracted from 

the diazinon concentrations determined in the surfacewater-spiked subsamples to give corrected results in 

tables 9, 10, and 11. No other method compounds were 

found in the surface-water sample, and no method 

compounds were detected in the reagent-water or 

ground-water samples. 

13.4 Estimated MDLs were determined by 

fortifying eight reagent-water samples, eight surfacewater samples, and eight ground-water samples, with 

the method compounds at 0.02 μg/L, a concentration 

that was twice the minimum reporting level of 0.01 

μg/L used in the previous method described by 

Wershaw and others (1987). The MDL was calculated 

by using the following equation:

where 

S = standard deviation of replicate analyses, in

 micrograms per liter, at the lowest

 concentration;

n = number of replicate analyses; and

 t(n - 1, 1− α = 0.99)

 = Student's t − value for the confidence level 

with n −1 degrees of freedom (U.S.

Environmental Protection Agency, 1997).

NOTE: Four significant figures after the decimal were 

used for standard deviations during MDL calculations.

13.5 Precision (percent relative standard 

deviation) and bias (percent mean recovery) for all 

matrices are listed in tables 6 through 14. Overall 

precision and bias of the compounds—combining all 

three water matrices and all nine concentrations—are 

listed in table 15. The term “variability” often is used 

interchangeably with the term “precision,” and 

“precision” is used in this report. Excellent performance is indicated for this method, with most 

compounds having relative standard deviations (RSD) 

less than 25 percent in all three matrices and mean 

recoveries in excess of 70 percent, especially at low 

concentrations. O-ethyl-O-methyl-S-propylphosphorothioate showed variable performance in all three 

matrices in comparison to the performance of all other 

compounds. It showed low RSD and good recovery 

(greater than 60 percent) for lower spikes in naturalwater samples, but poor recovery (less than 40 percent) 

in high spikes for all sample types at about 30 percent; 

it showed RSD less than 10 percent with good recovery 

(above 60 percent) for high spiked samples. 

13.6 The recoveries for methidathion, disulfoton 

sulfone, and ethion monoxon in surface-water samples 

were substantially greater than 100 percent (107 to 139 

percent), especially in low- and medium-level-spiked 

samples. It is possible that they are present in the 

surface-water source at levels near or less than the 

MDL and would contribute to the concentrations 

recovered. These results also could be caused by 

matrix-enhanced sensitivity. The injection of a 

complex matrix sample extract might coat the surfaces 

of the injection port with matrix components and 

protect compounds from decomposition or adsorption. 

As a result, a greater response is observed for 

compounds in sample extracts than in clean calibration 

solutions (Erney and others, 1993, 1997). MDL S t , (4) (, ) n – 1 1 – α= 0.99 = ×
Analytical Method 15

Table 6. Precision and bias from eight determinations of the method compounds 

spiked at 0.02 microgram per liter in reagent water for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter; 

ni, not in spike solution; na, not applicable] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.016 0.002 10.18 80.63 

Diazinon .018 .005 26.98 87.50 

Dimethoate .019 .002 9.58 92.50 

Disulfoton1 .013 .002 12.60 66.25 

 Disulfoton sulfone .021 .002 9.20 103.75 

 

Ethion .017 .002 9.43 85.00 

 Ethion monoxon1 .018 .002 8.91 90.00 

Ethoprop .016 .001 7.85 79.38 

 O-Ethyl-O-meth_ioate1 ni na na na 

Fenthion .016 .002 10.35 77.50 

 

Fonofos .015 .001 8.38 74.38 

 Fonofos oxon .017 .002 11.07 86.88 

Malathion .017 .002 10.17 86.88 

Methidathion .018 .002 8.70 91.88 

Methyl parathion .017 .002 10.62 86.25 

 

Parathion .015 .002 14.72 76.88 

Phorate .012 .002 12.15 61.25 

 Phorate oxon .015 .002 12.15 74.38 

Profenofos .018 .001 7.22 88.75 

Propetamphos .016 .001 8.34 78.13 

 

Sulfotepp .014 .002 12.07 70.00 

Sulprofos .016 .002 9.45 80.00 

Terbufos .014 .002 11.50 68.75 

Tribufos .017 .002 10.17 86.88 

Trithion .017 .002 10.71 84.38 

Surrogate 

Isofenfos 0.017 0.002 9.17 86.25 

1

Compound will be reported permanently as “E” coded (estimated concentration) 

in this method. 
16 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

Table 7. Precision and bias from eight determinations of the method compounds 

spiked at 0.05 microgram per liter in reagent water for the analytical method 

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter; 

ni, not in spike solution; na, not applicable] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent)

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.034 0.006 19.15 67.00 

Diazinon .036 .006 15.59 72.50 

Dimethoate .042 .005 11.43 83.50 

Disulfoton1

 .031 .003 10.75 61.25 

 Disulfoton sulfone .046 .007 14.20 91.75 

 

Ethion .038 .006 16.65 76.75 

 Ethion monoxon1 .041 .006 14.59 81.75 

Ethoprop .034 .005 14.62 68.50 

 O-Ethyl-O-meth_ioate1 ni na na na 

Fenthion .035 .006 16.59 70.50 

 

Fonofos .031 .005 14.93 62.00 

 Fonofos oxon .039 .005 13.27 77.25 

Malathion .039 .005 13.33 78.50 

Methidathion .039 .010 25.92 77.75 

Methyl parathion .038 .006 14.34 76.75 

 

Parathion .036 .006 16.69 72.75 

Phorate .025 .007 29.91 49.50 

 Phorate oxon .033 .005 16.45 65.00 

Profenofos .039 .008 19.96 78.00 

Propetamphos .034 .005 13.99 68.75 

 

Sulfotepp .030 .007 23.42 59.00 

Sulprofos .038 .006 14.47 76.75 

Terbufos .028 .008 30.38 55.50 

Tribufos .039 .007 17.10 77.50 

Trithion .038 .006 16.20 76.25 

Surrogate 

Isofenfos 0.040 0.006 14.59 79.50 

1

Compound will be reported permanently as “E” coded (estimated concentration) in 

this method. 
Analytical Method 17

Table 8. Precision and bias from six determinations of the method compounds 

spiked at 0.5 microgram per liter in reagent water for the analytical method 

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.404 0.044 11.01 80.77 

Diazinon .300 .064 21.28 59.93 

Dimethoate .340 .043 12.55 68.07 

Disulfoton1

 .213 .052 24.52 42.63 

 Disulfoton sulfone .438 .076 17.45 87.60 

 

Ethion .212 .010 4.86 42.30 

 Ethion monoxon1

 .247 .017 7.02 49.30 

Ethoprop .260 .016 5.99 52.03 

 O-Ethyl-O-meth_ioate1

 .136 .013 9.82 27.17 

Fenthion .397 .048 12.02 79.33 

 

Fonofos .309 .052 16.73 61.73 

 Fonofos oxon .395 .029 7.38 79.07 

Malathion .394 .095 24.02 78.87 

Methidathion .397 .053 13.24 79.37 

Methyl parathion .410 .054 13.28 81.90 

 

Parathion .403 .030 7.47 80.57 

Phorate .223 .056 25.02 44.57 

 Phorate oxon .226 .037 16.15 45.23 

Profenofos .386 .072 18.74 77.10 

Propetamphos .347 .034 9.70 69.33 

 

Sulfotepp .215 .011 5.14 43.07 

Sulprofos .297 .016 5.44 59.40 

Terbufos .234 .061 25.95 46.77 

Tribufos .356 .017 4.67 71.13 

Trithion .292 .042 14.33 58.37 

Surrogate 

Isofenfos 78.288 8.604 10.99 78.29 

1

Compound will be reported permanently as “E” coded (estimated concentration) 

in this method. 
18 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

Table 9. Precision and bias from eight determinations of the method compounds 

spiked at 0.02 microgram per liter in surface water (South Platte River near 

Dartmouth St. and Platte River Dr., Denver, Colo.) for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.015 0.001 8.40 76.25 

Diazinon2 .015 .002 10.27 75.63 

Dimethoate .024 .001 5.40 118.75 

Disulfoton1

 .018 .002 9.16 87.50 

 Disulfoton sulfone .027 .002 8.27 136.88 

 

Ethion .014 .002 10.44 71.25 

 Ethion monoxon1

 .023 .002 8.05 115.00 

Ethoprop .017 .001 6.58 85.63 

 O-Ethyl-O-meth_ioate1

 .016 .001 4.04 79.38 

Fenthion .016 .002 12.43 77.50 

 

Fonofos .016 .001 7.25 81.88 

 Fonofos oxon .023 .002 8.16 113.13 

Malathion .019 .001 7.21 96.25 

Methidathion .023 .002 9.90 113.75 

Methyl parathion .019 .001 6.84 93.75 

 

Parathion .016 .001 9.12 77.50 

Phorate .016 .001 6.79 78.13 

 Phorate oxon .017 .001 7.43 86.25 

Profenofos .021 .002 8.17 105.63 

Propetamphos .016 .001 5.97 77.50 

 

Sulfotepp .014 .001 6.86 67.50 

Sulprofos .014 .002 13.59 69.38 

Terbufos .016 .001 7.09 79.38 

Tribufos .016 .001 9.12 77.50 

Trithion .016 .002 11.82 78.13 

Surrogate 

Isofenfos 0.017 0.001 6.29 85.00 

1

Compound will be reported permanently as “E” coded (estimated concentration) 

in this method. 2

Mean observed concentration after subtracting background diazinon concentration 

of 0.008 g/L. 
Analytical Method 19

Table 10. Precision and bias from eight determinations of the method compounds 

spiked at 0.05 microgram per liter in surface water (South Platte River near 

Dartmouth St. and near Platte River Dr., Denver, Colo.) for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.034 0.003 10.16 67.75 

Diazinon2 .037 .004 11.25 73.00 

Dimethoate .053 .006 12.07 105.75 

Disulfoton1

 .043 .005 10.52 85.50 

 Disulfoton sulfone .067 .008 12.15 134.50 

 

Ethion .030 .003 11.38 59.75 

 Ethion monoxon1

 .056 .006 9.94 111.25 

Ethoprop .040 .003 7.83 80.50 

 O-Ethyl-O-meth_ioate1

 .034 .003 8.82 68.75 

Fenthion .034 .003 8.82 67.75 

 

Fonofos .039 .003 7.79 78.50 

 Fonofos oxon .054 .005 9.29 107.25 

Malathion .045 .005 10.89 90.00 

Methidathion .054 .004 8.03 107.25 

Methyl parathion .044 .003 7.02 88.75 

 

Parathion .032 .003 7.85 63.50 

Phorate .037 .004 10.02 74.50 

 Phorate oxon .040 .005 11.87 80.25 

Profenofos .051 .005 10.42 101.00 

Propetamphos .037 .003 8.69 74.75 

 

Sulfotepp .035 .004 11.59 69.00 

Sulprofos .027 .004 15.06 54.25 

Terbufos .037 .004 11.72 73.00 

Tribufos .033 .004 11.09 66.75 

Trithion .030 .004 13.21 60.00 

Surrogate 

Isofenfos 0.039 0.004 9.67 77.75 

1

Compound will be reported permanently as “E” coded (estimated concentration) 

in this method 

2

Mean observed concentration after subtracting background diazinon concentration 

 	
20 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

Table 11. Precision and bias from eight determinations of the method compounds 

spiked at 0.5 microgram per liter in surface water (South Platte River near 

Dartmouth St. and Platte River Dr., Denver, Colo.) for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter; 

ni, not in spike solution; na, not applicable] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.358 0.026 7.25 71.55 

Diazinon2 .409 .031 7.49 81.75 

Dimethoate .521 .027 5.17 104.18 

Disulfoton1

 .354 .025 6.95 70.83 

 Disulfoton sulfone .560 .025 4.53 112.08 

 

Ethion .336 .020 6.05 67.13 

 Ethion monoxon1

 .318 .004 1.34 63.63 

Ethoprop ni na na na 

 O-Ethyl-O-meth_ioate1

 .172 .004 2.48 34.35 

Fenthion .409 .022 5.38 81.85 

 

Fonofos .353 .030 8.42 70.68 

 Fonofos oxon .474 .023 4.94 94.70 

Malathion .442 .020 4.62 88.38 

Methidathion .477 .021 4.30 95.40 

Methyl parathion .457 .019 4.19 91.33 

 

Parathion .435 .020 4.59 87.03 

Phorate .332 .027 8.04 66.40 

 Phorate oxon .423 .033 7.86 84.68 

Profenofos .452 .023 5.12 90.45 

Propetamphos .369 .022 5.91 73.70 

 

Sulfotepp .353 .015 4.30 70.68 

Sulprofos .327 .026 7.94 65.30 

Terbufos .417 .024 5.72 83.33 

Tribufos .373 .025 6.69 74.53 

Trithion .358 .023 6.39 71.50 

Surrogate 

Isofenfos 54.303 3.074 5.66 54.30 

1

Compound will be reported permanently as “E” coded (estimated concentration) in 

this method. 

2

Mean observed concentration after subtracting background diazinon concentration 

of 0.008 	. 
Analytical Method 21

Table 12. Precision and bias from eight determinations of the method 

compounds spiked at 0.02 microgram per liter in ground water (domestic 

well water, Evergreen, Colo.) for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.015 0.001 5.09 73.13 

Diazinon .016 .001 7.47 80.00 

Dimethoate .021 .001 4.00 104.38 

Disulfoton1

 .011 .004 34.15 52.50 

 Disulfoton sulfone .022 .002 8.06 108.75 

 

Ethion .015 .001 4.31 74.38 

 Ethion monoxon1

 .020 .001 5.21 101.88 

Ethoprop .015 .001 7.44 75.63 

 O-Ethyl-O-meth_ioate1

 .014 .002 14.29 67.50 

Fenthion .014 .002 14.29 67.50 

 

Fonofos .015 .001 9.42 73.75 

 Fonofos oxon .020 .002 7.75 97.50 

Malathion .017 .001 4.28 86.88 

Methidathion .020 .001 6.19 100.63 

Methyl parathion .017 .001 4.87 85.63 

 

Parathion .015 .001 6.26 73.13 

Phorate .013 .001 6.36 65.63 

 Phorate oxon .014 .002 11.20 67.50 

Profenofos .019 .001 6.29 95.00 

Propetamphos .014 .001 8.12 69.38 

 

Sulfotepp .013 .001 6.69 66.25 

Sulprofos .014 .001 5.46 68.13 

Terbufos .013 .001 8.22 65.00 

Tribufos .015 .001 4.24 75.63 

Trithion .016 .001 4.76 78.13 

Surrogate 

Isofenfos 0.016 0.001 5.63 78.75 

1

Compound will be reported permanently as “E” coded (estimated concentration) 

in this method. 
22 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

Table 13. Precision and bias from eight determinations of the method compounds 

spiked at 0.05 microgram per liter in ground water (domestic well water, Evergreen, 

Colo.) for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram per liter]

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.035 0.006 16.62 69.50 

Diazinon .037 .005 13.82 73.00 

Dimethoate .047 .007 15.58 94.50 

Disulfoton1

 .034 .009 26.65 67.00 

 Disulfoton sulfone .051 .008 16.10 102.00 

 

Ethion .037 .005 12.26 74.00 

 Ethion monoxon1

 .048 .007 14.23 95.00 

Ethoprop .036 .005 14.78 71.50 

 O-Ethyl-O-meth_ioate1

 .030 .004 14.46 59.50 

Fenthion .034 .007 19.35 67.00 

 

Fonofos .034 .005 13.49 68.50 

 Fonofos oxon .045 .006 13.40 90.50 

Malathion .040 .007 16.25 80.50 

Methidathion .046 .006 13.47 92.50 

Methyl parathion .040 .006 13.94 80.75 

 

Parathion .034 .005 15.47 68.75 

Phorate .032 .006 17.31 63.25 

 Phorate oxon .033 .007 21.61 66.00 

Profenofos .044 .007 14.70 88.75 

Propetamphos .034 .005 13.97 67.75 

 

Sulfotepp .030 .004 14.00 60.50 

Sulprofos .035 .005 14.47 70.25 

Terbufos .033 .006 17.56 65.00 

Tribufos .038 .004 11.50 75.75 

Trithion .038 .005 12.22 76.75 

Surrogate 

Isofenfos 0.039 0.005 13.39 77.00 

1

Compound will be reported permanently as “E” coded (estimated concentration) in 

this method. 
Analytical Method 23

Table 14. Precision and bias from eight determinations of the method compounds 

spiked at 0.5 microgram per liter in ground water (domestic well water, Evergreen, 

Colo.) for the analytical method

[Compound degradates are indented. conc., concentration; μg/L, microgram 

per liter; ni, not in spike solution; na, not applicable] 

Compound 

Mean 

observed 

conc. 

(μg/L) 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 0.325 0.046 14.09 65.03 

Diazinon .353 .049 13.99 70.68 

Dimethoate .442 .061 13.68 88.38 

Disulfoton1

 .202 .046 23.02 40.33 

 Disulfoton sulfone .479 .064 13.45 95.70 

 

Ethion .305 .039 12.91 60.95 

 Ethion monoxon1

 .362 .066 18.29 72.40 

Ethoprop ni na na na 

 O-Ethyl-O-meth_ioate1

 .170 .016 9.26 33.90 

Fenthion .344 .047 13.77 68.85 

 

Fonofos .310 .052 16.62 61.95 

 Fonofos oxon .414 .060 14.48 82.85 

Malathion .387 .053 13.64 77.33 

Methidathion .417 .055 13.17 83.30 

Methyl parathion .401 .056 14.04 80.10 

 

Parathion .381 .052 13.60 76.18 

Phorate .268 .054 19.98 53.68 

 Phorate oxon .345 .053 15.44 69.03 

Profenofos .402 .054 13.52 80.30 

Propetamphos .307 .050 16.18 61.33 

 

Sulfotepp .313 .054 17.32 62.60 

Sulprofos .299 .043 14.34 59.75 

Terbufos .345 .050 14.46 68.95 

Tribufos .340 .046 13.40 67.98 

Trithion .325 .044 13.59 65.03 

Surrogate 

Isofenfos 50.835 5.618 11.05 50.84 

1

Compound will be reported permanently as “E” coded (estimated concentration) 

in this method.
24 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

Table 15. Overall precision and bias combined from the determinations of the method 

compounds spiked at 0.02, 0.05, and 0.5 microgram per liter in reagent water, surface 

water (South Platte River near Dartmouth St. and Platte River Dr., Denver, Colo.), and 

ground water (domestic well water, Evergreen, Colo.) for the analytical method

[Compound degradates are indented. μg/L, microgram per liter, conc., concentration] 

Compound Number of

observations 

Standard 

deviation 

(μg/L) 

Relative 

standard 

deviation 

(percent) 

Bias 

(percent 

of mean 

conc.) 

Chlorpyrifos 70 9.66 13.89 72.16 

Diazinon 70 13.10 17.40 75.31 

Dimethoate 70 16.31 16.93 96.34 

Disulfoton1

 70 18.86 29.31 64.38 

 Disulfoton sulfone 70 20.21 18.59 108.70 

 

Ethion 70 13.20 19.22 68.68 

 Ethion monoxon1

 70 22.13 25.22 87.78 

Ethoprop 54 11.80 15.93 74.10 

 O-Ethyl-O-meth_ioate1

 54 19.76 36.67 53.89 

Fenthion 70 10.27 14.09 72.91 

 

Fonofos 70 10.25 14.51 70.62 

 Fonofos oxon 70 14.64 15.83 92.50 

Malathion 70 11.33 13.33 85.01 

Methidathion 70 15.62 16.62 93.94 

Methyl parathion 70 9.77 11.48 85.11 

 

Parathion 70 10.29 13.72 74.99 

Phorate 70 13.36 21.43 62.37 

 Phorate oxon 70 14.34 20.02 71.68 

Profenofos 70 13.61 15.16 89.79 

Propetamphos 70 8.61 12.08 71.23 

 

Sulfotepp 70 10.71 16.79 63.75 

Sulprofos 70 10.95 16.28 67.24 

Terbufos 70 13.77 20.29 67.88 

Tribufos 70 9.47 12.64 74.95 

Trithion 70 11.56 15.96 72.45 

1

Compound will be reported permanently as “E” coded (estimated concentration) in this 

method. 
Summary and Conclusions 25

13.7 Excellent performance is indicated for most 

compounds with RSD less than 25 percent (except 

disulfoton, ethion monoxon, and O-ethyl-O-methyl-Spropylphosphorothioate) and mean recoveries in 

excess of 60 percent (except O-ethyl-O-methyl-Spropylphosphorothioate) in all three matrices (table 

15). Disulfoton and malathion, which have been poorperforming compounds in the whole-water method 

(Wershaw and others, 1987), demonstrated good 

precision and recoveries (greater than 60 percent) in all 

matrices tested at three different concentrations in this 

new method. Because disulfoton and ethion monoxon 

have RSDs greater than 25 percent (table 15), they will 

be reported permanently with an estimated remark 

code. Also, O-ethyl-O-methyl-S-propylphosphorothioate has recovery less than 60 percent and 

RSD greater than 25 percent; therefore, this compound 

will be reported permanently with an estimated remark 

code. These criteria include estimated recoveries 

greater than 120 or less than 60 percent and RSD 

greater than 25 percent (Sandstrom and others, 2001; 

Furlong and others, 2001).

13.8 All compounds listed in table 16 were deleted 

from the method because of unacceptable method 

performance.

Not enough research was done on these 

compounds to clarify the reason for unacceptable 

method performance. Some compounds did not 

respond because of nondetectability by GC/FPD and 

low solubility in hexane, the solvent used to prepare the 

initial GC/FPD evaluation standard. In addition, 

photodecomposition or rapid degradation in water, 

volatility (excessive volatilization losses during sample 

preparation), and thermal liability could be other 

reasons for their unacceptable performance.

NOTE: All the precision and bias data for this method 

are for compounds added to whole-water samples in 

the dissolved form. Recoveries for compounds 

absorbed to particulate matter in samples have not been 

determined. Recoveries from particulate matter may be 

lower and more variable than recoveries from the 

dissolved phase.

SUMMARY AND CONCLUSIONS

Previously, the National Water Quality Laboratory 

(NWQL) determined organophosphate pesticides 

recoverable in whole water by using the U.S. 

Geological Survey methods described by Wershaw and 

others (1987). The original method included 11 compounds. This new whole-water method uses a 

continuous liquid-liquid extraction procedure that 

improves organophosphate pesticide recoveries. It also 

expands the selected list of determined compounds 

from 11 to 25. 

This report presents a method for the routine 

analysis of 20 parent organophosphate pesticides and 5 

pesticide degradates in whole-water samples that 

include three compounds (disulfoton, ethion monoxon, 

and O-ethyl-O-methyl-S-propylphosphorothioate) to 

be reported permanently as estimated (E-coded) 

concentration. Mean recoveries of most method 

compounds spiked in surface-water samples ranged 

from 54 to 137 percent and those in ground-water 

samples ranged from 40 to 109 percent for all pesticides. Recoveries of method compounds, except Oethyl-O-methyl-S-propylphosphorothioate, in spiked 

reagent-water samples ranged from 42 to 104 percent. 

O-ethyl-O-methyl-S-propylphosphorothioate was the 

only compound that showed low recovery (27 percent). 

Single-operator method detection limits (determined 

and combined in all three matrices) ranged from 0.003 

to 0.009 μg/L. Malathion and disulfoton, which have 

been poor-performing compounds (recoveries less than 

30 percent on average) in the whole-water method 

(Wershaw and others, 1987), demonstrated good 

precision and recoveries (greater than 60 percent on 

average) in all matrices tested at three different 

concentrations in this new whole-water method.

Table 16. Compounds tested and found to have 

unacceptable performance

Compound name 

Acephate 

Azinphos methyl oxon 

Cadusaphos 

Chlorpyrifos oxon 

Diazinon oxon 

Dichlorovos 

Dichrotophos 

Dimethoate oxon 

Fenamiphos 

Fenamiphos sulfone 

Fenamiphos sulfoxide 

Fenthion sulfone 

Guthion 

Malaoxon 

Metamidophos 

Naled 

Paraoxon ethyl 

Paraoxon methyl 

Phosmet 

Temephos 

Temephos sulfoxide 

Terbufos oxygen analog sulfone 
26 DETERMINATION OF ORGANOPHOSPHATE PESTICIDES IN WHOLE WATER BY CONTINUOUS LIQUID-LIQUID 

EXTRACTION AND CAPILLARY-COLUMN GAS CHROMATOGRAPHY WITH FLAME PHOTOMETRIC DETECTION

REFERENCES CITED

Childress, C.J.O., Foreman, W.T., Connor, B.F., and 

Maloney, T.J., 1999, New reporting procedures 

based on long-term method detection levels and 

some considerations for interpretations of waterquality data provided by the U.S. Geological 

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